Implantable medical devices, including cardiac rhythm management devices such as pacemakers and implantable cardioverter/defibrillators, usually have the capability to communicate data with a device called an external programmer via a radio frequency telemetry link. A clinician may use an external programmer to program the operating parameters of an implanted medical device. For example, the pacing mode and other operating characteristics of a pacemaker may be modified after implantation in this manner. Modern implantable devices also include the capability for bidirectional communication so that information can be transmitted to the programmer from the implanted device. Among the data which may be telemetered from an implantable device are various operating parameters and physiological data, the latter either collected in real-time or stored from previous monitoring operations.
Telemetry systems for implantable medical devices utilize radio frequency energy to enable bidirectional communication between the implantable device and an external programmer. A radio frequency carrier is modulated with digital information by, for example, amplitude shift keying where the presence or absence of pulses in the signal constitute binary symbols or bits. The external programmer transmits and receives the radio signal with an antenna incorporated into a wand which can be positioned in proximity to the implanted device. The implantable device also generates and receives the radio signal by means of an antenna formed by a wire coil wrapped around the periphery of the inside of the device casing.
In previous telemetry systems, the implantable device and the external programmer communicate by generating and sensing a modulated electromagnetic field in the near-field region with the antennas of the respective devices inductively coupled together. The wand must therefore be in close proximity to the implantable device, typically within a few inches, in order for communications to take place. This requirement is inconvenient for a clinician and limits the situations in which telemetry can take place.